It was in 2003, when he was a promising 16-year-old who had yet to break into the world's top 100, that Rafael Nadal last lost a match here at the Monte Carlo Masters. Since that defeat, to Guillermo Coria, the Spaniard has won 27 matches in succession at the Monte Carlo Country Club.

As Nadal begins his attempt to claim a sixth successive title on these courts – he meets the Dutchman Thiemo de Bakker this afternoon – it is a record that should fill the world No 3 with confidence and his opponents with fear. However, as the big-name players enter the fray at the traditional start of the European outdoor season, one question dominates: is the king of clay about to be overthrown?

It could be argued that defeat to Robin Soderling at last year's French Open marked the end of Nadal's reign on terre battue, but at the time it seemed that the Majorcan had lost largely because his suspect knees had finally succumbed to the demands of his all-action game.

When he was back to full health, it was argued, Nadal would inevitably return to the form that had seen him dominate on clay for more than four years. In 154 contests on his favourite surface between April 2005 and May 2009 Nadal lost only four times – at one stage he won 81 matches in a row – while his defeat to Soderling ended an extraordinary run of 48 consecutive wins in five-set matches on clay.

However, Nadal has been back playing regularly for eight months – despite brief absences with a stomach muscle injury and another knee problem, he has played in 11 events and the Davis Cup final since making his comeback in August – without adding to his career tally of 36 titles.

Nadal last celebrated victory more than 11 months ago in Rome, which, along with Monte Carlo and Barcelona, is one of the remaining three titles he holds. Perhaps even more worryingly, the 23-year-old Spaniard has lost 12 of his last 14 matches against opponents from the world's top 10, his only victories coming against Jo-Wilfried Tsonga at the Paris Masters in November and in Miami a fortnight ago.

Although Nadal agrees that he was "not ready to win important tournaments" last year, he believes his recent form – including semi-final appearances in the Masters events in Indian Wells and Miami – shows that he is ready to win again. He has never been one to talk up his own chances – "I've never felt invincible on any surface, even clay," he said here – but is keen to stress how well he played on clay throughout the season last year. "I lost one match at Roland Garros," he says, and indeed it was the only match he has ever lost at the French Open, "and one final in Madrid [against Roger Federer]. I'd won the last I don't know how many tournaments on clay. I won the Davis Cup final on clay, including an important match for me against [Tomas] Berdych.

"I'll be trying to play my best to win here, but this is a very difficult tournament, with nearly all the world's best players. I hope I'll do as well as I have in the past, but just because I've won Monte Carlo the last five years in a row doesn't mean that I will win for a sixth time."

Nadal brushes aside doubts about his fitness and always rejects the idea that he is less muscular than he used to be, the suggestion having been floated last year that he might have been advised to lose weight because of the demands on his knees. What is indisputable is that Nadal's all-action style, dogged defence and refusal to believe that any cause is lost have placed huge stresses on his body. More attacking players like Federer generally expend far less energy in winning points than counter-punchers like the Spaniard.

For whatever reason, some degree of energy appears to have gone from Nadal's game over the last year. While his pounding ground strokes remain a potent weapon, the competition at the top of men's tennis is so ferocious that his failure to intimidate opponents with the sheer physical force of his game has given encouragement to others. Nadal sees no point in discussing what others might make of his situation – "You can ask them, not me," he said – but some of his rivals clearly believe there has been a swing in the balance of power.

"Other players now have more belief that they can win against Nadal on clay," Novak Djokovic said. "He has been so dominant on this surface, but everybody knew that the moment would come, sooner or later, where he would drop his level of performance a little bit and where some other players would have a chance to win. That's what has happened."

Djokovic believes that Nadal's problems may have stemmed from their extraordinary four-hour semi-final in Madrid last year. "If you look at the tournaments we played after that neither of us fully recovered 100 per cent physically or mentally," the world No 2 said. "He got injured and missed the whole of the grass-court season."

Fernando Verdasco agreed that his Davis Cup team-mate had been all but unbeatable on clay at this stage of the season last year. "Everybody knew it was going to be hard because he was running for every ball, putting every ball in," Verdasco said. "He moved you, never gave you the chance to advance in the points. It was almost impossible to beat him."

Verdasco conceded that Nadal might have lost a little confidence over the last year but stressed: "I think he still believes that he's the best on clay. His mentality is different on clay. I think he's still going to be the guy to beat here."

Djokovic agreed. "Look, he still has the best record ever on clay," he said. "In the end he only lost two matches on this surface last year, against Federer and Soderling, when he wasn't playing his best. He's still the No 1 favourite for every tournament on clay."

Anatomy of a weakened champion: Where Nadal can improve

Mental strength

Nadal, who has won so many tight matches over the years, has a reputation as one of the game's toughest players mentally, but this clay-court season offers his greatest challenge yet. Over the next month he defends the only three titles he still holds, in Monte Carlo, Barcelona and Rome.

Knees

Nadal plays an intensely physical game, which inevitably puts more stress on his suspect knees. They finally gave way last summer, forcing him to miss Wimbledon, while a new problem led to his retirement against Andy Murray at the Australian Open.

Serve

The lack of a killer serve has rarely been a problem for Nadal in the past, but when he is below his best it denies him the flow of free points that can be a bonus for big-hitting rivals.

Physical strength

Nadal consistently denies that he has lost bulk – he suggests he may look less muscular simply since he ditched his trademark sleeveless shirts – but the impression remains that he is not as intimidating physically as he used to be.

Movement

Perhaps wary of his knee problems, Nadal has appeared less inclined to chase down every single ball in the way that he used to. His extraordinary defensive abilities were arguably his greatest strength but have recently been called into question.